It is often necessary or desirable to transfer data between data processing systems, such as digital processing systems. Typically, the transfer of data requires a modem or other interface to convert from the digital data on a digital processing system to data in other formats, such as analog formats. A typical example involves the use of a computer system with a modem at one location to transfer a data file from that computer system to a remotely located computer system which has its own modem or interface. The transfer of large data files, such as digital pictures, requires considerable time even with high-speed analog modems. The use of other types of modems is less common, although these modems do tend to improve the speed of file transfers between computer systems. Still, considerable time can be required to transfer large files even with these other types of modems.
Typically, the two computer systems establish a communication link and then one computer system begins sending a file or other data to the other computer system. Sometimes, the data is encrypted. Also, sometimes, a hashed or digested version of the file may be transferred with the file in order to allow the recipient of the file to authenticate the validity of the file as received at the second computer system.
These prior approaches to transferring data often result in the transmission of unnecessary data between the computer systems. As a result, considerable time can be spent transferring unneeded data. For example, a file could be transferred from a source computer system to a recipient computer system while that same file already exists on the recipient computer system as no attempt is made to determine whether the recipient already has the file. It should be noted that one solution may involve checking for the file name on the recipient system before sending the file; however, file names can be changed so that a check for the file name may not yield the correct answer. Moreover, even if the file names are not changed, it is still possible to have the same file names for different files by coincidence.
Thus it is desirable to provide a method and apparatus for eliminating the transfer of data between digital processing systems when the transfer is not necessary.